Why did Bernie Sanders dominate Saturday? Caucuses in whiter states.

Sen. Bernie Sanders had the best night of his presidential campaign on Saturday, dominating Democratic rival Hillary Clinton in the Washington and Alaska state caucuses by wide margins. (As of writing, the results in Hawaii have not been determined.) He cut into Clinton's pledged-delegate lead by at least one-sixth and potentially more. It was the sort of night that he needs more of.

But which he's almost certainly not going to get.

The reason it was such a big night for Sanders was that he dominated in Washington state, beating Clinton by more than 40 points. Washington has a big delegate total, so splitting up the delegates gave Sanders a big margin. His giant win in Alaska was icing on that cake.

But Alaska and Washington had two characteristics that made them very friendly terrain for Sanders: They were caucuses in predominantly non-black states. And there aren't many more of those on the calendar.

Clinton has done worse in caucuses in both of her two presidential bids. In 2008, Clinton's median margin of victory in primaries was about a point; her median loss to Barack Obama that year in caucuses was about 34 points.

Even including Sanders' blow-out in the Vermont primary this year, there's an even wider gulf. Her median victory in primaries has been 23 points, and Sanders' median victory in caucuses has been 26 points (using nonfinal numbers in Alaska and Washington).

As we've noted before, there's also a clear link between the number of black voters in a contest and the result. Alaska is four percent black; Washington, about the same. When the composition of the black Democratic electorate has been below seven percent for states where Democratic primary exit polling in 2008 or 2016 was available, Clinton has lost by an average of 30 points this year. Over that percentage? She's won by 26.

There are still three more caucuses on the Democratic calendar, all of them very small contests: Wyoming, Guam and the Virgin Islands. Also left on the calendar? A lot of big, diverse states holding primaries. Washington and Alaska were caucus states with small black populations. That's as good as it gets for Bernie Sanders.